1. Field of the Invention
This invention relates to a reactive injection molding (RIM) process for manufacturing a two-colored molded polyurethane product having a surface portion and an inner portion differing from each other in color and properties. The invention also relates to a reactive injection molding process for manufacturing a molded polyurethane product containing a metal core.
2. Description of the Related Art
The assignee of the inventor of this invention has proposed a RIM process for manufacturing a molded product of polyurethanes as shown in FIGS. 42 and 43 (see Japanese Patent Application Laid-Open Specification No. 6-143340). The following is an outline of the process:
(1) The upper and lower portions 102 and 103 of a mold 100 are separated from each other, and a metal core 110 for a steering wheel is placed in its annular cavity 104. The upper and lower mold portions 102 and 103 are joined together, and a support pin 105 is moved to hold the metal core 110 in position; PA1 (2) A vacuum pump 106 is operated to evacuate a vacuum casing 108, and the cavity 104 through a vent hole 107 and a clearance between the surfaces 102a and 103a of the mold portions mating each other along their parting line; PA1 (3) A surface-forming polyurethane material U1 is injected into the cavity 104 having a reduced pressure. A sudden pressure drop to which the polyurethane material U1 having an elevated pressure is subjected causes it to scatter in the cavity 104 and adhere to substantially the entire wall surface surrounding the cavity 104 as a film forming the surface portion 112 of a polyurethane covering 111 for the steering wheel; PA1 (4) An inner-forming polyurethane material U2 is injected into the cavity 104 remaining at a reduced pressure. The polyurethane material U2 undergoes foaming, flows in two directions to fill the cavity 104, and undergoes reactive curing to form the inner portion 113 of the polyurethane covering 111. A small amount of the polyurethane material U2 at the leading ends of its streams, as well as the material U1, flows out through the vent hole 107 and forms a solidified outflow 114, as shown in FIG. 42; PA1 (5) The upper and lower mold portions 102 and 103 are separated from each other, and the polyurethane covering 111 containing the metal core 110 is removed from the cavity, as shown in FIG. 43. The outflow 114 which is separated from a vent-hole burr 115 formed in the vent hole 107 on that occasion is removed from the upper mold portion 102. The burr 115 adhering to the polyurethane covering 111 is manually cut off, as shown in FIG. 44. PA1 (1) The surface-forming polyurethane material has a higher rate of urethane reaction than the inner-forming one; PA1 (2) Extra time is allowed for the surface-forming polyurethane material, injected during the step of injecting a surface-forming polyurethane material, to cure between the steps of injecting the surface-forming polyurethane material and the inner-forming polyurethane material; and PA1 (3) The step of injecting the surface-forming polyurethane material is divided into a first injection step and a second injection step, a step of extra time for allowing the surface-forming polyurethane material, injected during the first injection step, to cure intervenes between the first and second injection steps. PA1 (a) The surface-forming polyurethane material may contain coloring matter, while the inner-forming one does not; PA1 (b) The surface-forming polyurethane material can contain a mold release agent, while the inner-forming one does not; PA1 (c) The surface-forming polyurethane material can contain a catalyst for modifying the properties, while the inner-forming one does not; PA1 (d) The surface-forming polyurethane material can have a higher rate of urethane reaction than the inner-forming one; and PA1 (e) The surface-forming polyurethane material may be one which does not undergo yellowing, while the inner-forming one does.
The known process as outlined above facilitates the manufacture of a two-colored molded product of polyurethanes having a thin surface portion covering its inner portion, such as the polyurethane covering 111.
The process has a number of advantages. For example, it is possible to eliminate the work of applying a mold release agent if the surface portion 112 contains such an agent. It is possible to eliminate the work of applying coloring material if the surface portion 112 contains such a material. It is also possible to employ an appropriate catalyst in the surface portion 112 to give it a wide range of properties, such as touch and durability, differing from those of the inner portion 113.
Moreover, the process does not give any product having a defective appearance, if it is possible to remove through the vent hole 107 those parts of the inner-forming polyurethane material U2 at the leading ends of its streams which are unnecessary as they contain an excess of bubbles (composed of reactive gases, such as air and carbon dioxide). The process has, however, a number of drawbacks, too, as stated at (1) to (4) below.
(1) The polyurethane material U2 remaining in a gate 109 forms a gate burr 116 adhering to the polyurethane covering 111, as shown in FIG. 43. The burr 116 has to be cut off, but when it is cut off, the polyurethane covering 111 has a cut section 118 in which its inner portion 113 is exposed. If the polyurethane material U2 does not contain any coloring matter, while the surface-forming polyurethane material U1 contains one, the exposure of the uncolored inner portion 113 in the cut section 118 spoils the appearance of the covering 111.
(2) Although the surface-forming polyurethane material U1 adhering to the wall of the cavity 104, as stated at (3) above, may become somewhat higher in viscosity, it is still so flowable or moldable when the inner-forming polyurethane material U2 is injected, as stated at (4) above, that the streams of the polyurethane material U2 may cause a gradual flow of the polyurethane material U1 and thereby an undesirable reduction in thickness of the surface portion 112. This is particularly the case with the surface portion 112 formed in the vicinity of the gate 109 where it is affected by the polyurethane material U2 for a longer time than anywhere else.
If the polyurethane material U2 does not contain any coloring matter, while the polyurethane material U1 does, the reduction in thickness, as shown by broken lines, of the surface portion 112 in the vicinity of the gate 109 results in the exposure of the uncolored inner portion 113, or the unevenness in color density of the surface portion 112, as shown in FIGS. 46 and 47, and thereby damages the outward appearance of the covering 111.
If the polyurethane material U2 does not contain any mold release agent, while the polyurethane material U1 does, the reduction in thickness of the surface portion 112 in the vicinity of the gate 109 lowers the mold releasability of the product in that region.
If the polyurethane material U2 does not contain any catalyst, while the polyurethane material U1 does, the reduction in thickness of the surface portion 112 in the vicinity of the gate 109 lowers the properties of the product in that region.
If the inner-forming polyurethane material U2 is a common inexpensive one, while the surface-forming polyurethane material U1 is an expensive material of high performance (e.g. a non-yellowing one), the reduction in thickness of the surface portion 112 in the vicinity of the gate 109 makes the product fail to satisfy the standard for performance in that region.
(3) As the polyurethane material is allowed to flow out through the vent hole 107 at the leading ends of its streams, it is necessary before or after each cycle of molding operation to supply the vent hole 107 with a mold release agent for facilitating the removal of the burr 115 therefrom, to remove the outflow 114 from the upper mold portion 102, and to cut off the burr 115 manually from the covering 111. These additional jobs have lowered the efficiency of each cycle of molding operation. A high level of skill has been required for, above all, cutting off the burr 115 neatly, and however neatly it may have been cut off, a cutting mark 119 made on the covering 111 has spoiled its appearance and touch.
(4) The polyurethane material U1 is driven by the leading ends of the streams of the polyurethane material U2 flowing in two directions from the gate 109, as stated at (2) above, and gathers in a final filling portion 117 leading to the vent hole 107, as shown in FIGS. 46 and 48. If the polyurethane material U1 which has gathered in the final filling portion 117 stays there without flowing out through the vent hole 107, it has been likely that the difference in degree of shrinkage between the materials in the final filling portion 117 and anywhere else may result in a product lacking uniformity in outside diameter, touch, etc. Accordingly, it has also been likely that the polyurethane material U2 containing an excess of bubbles at the leading ends of its streams may not be allowed to flow out satisfactorily through the vent hole 107, resulting in a product having an outward appearance which is defective for pinholes. In order to overcome these problems, it has been necessary for the outflow 114 to have a weight of, say, 15 to 30 g.
Japanese Utility Model Laid-Open Specification Nos. 52-169233 and 52-169234 disclose a steering wheel including a covering having a portion fitted in a hollow portion of a metal core. This device is, however, intended for holding the covering against rotation about the core, and the covering is made by molding at normal pressure. Japanese Utility Model Laid-Open Specification No. 59-172067 discloses a steering wheel having a covering filling a hollow portion of a metal core. This device is, however, intended for reinforcing the core, and the covering is made by molding at normal pressure. Thus, none of their disclosures has anything to do with the technical concept of this invention.